1. Field of the Invention
This invention relates generally to perpendicular magnetic recording (PMR) systems, and more particularly to a PMR system with a spin-torque oscillator (STO) that provides microwave-assisted magnetic recording (MAMR).
2. Description of the Related Art
Perpendicular magnetic recording (PMR) in magnetic recording hard disk drives, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the magnetic recording layer of the disk, allows for ultra-high recording density, i.e., the areal density of the recorded bits on the disk. However, an increase in recording density requires a corresponding reduction in the size of the magnetic grains in the magnetic recording layer to achieve sufficient medium signal-to-noise ratio. As the size of the magnetic grains is reduced, the magneto crystalline anisotropy of the magnetic grains must be increased to maintain adequate thermal stability. Simultaneously, the magnetic write field from the write head has to exceed the coercivity of the magnetic recording layer to achieve saturation digital recording, resulting in a conflicted limitation on the anisotropy of the magnetic grains.
A PMR system with high-frequency assisted writing using a spin-torque oscillator (STO) has been proposed. This type of recording, also called microwave-assisted magnetic recording (MAMR), applies a high frequency oscillatory auxiliary magnetic field from the STO to the magnetic grains of the recording layer. The auxiliary field may have a frequency close to the resonance frequency of the magnetic grains in the recording layer to facilitate the switching of the magnetization of the grains at lower write fields from the conventional write head than would otherwise be possible without assisted recording. Conversely, MAMR may be used to increase the coercivity of the magnetic recording layer above that which could be written to by a conventional PMR alone. The increase in coercivity afforded by MAMR allows for a reduction in the size of the magnetic grains and thus a corresponding increase in recording density system. MAMR systems are described in U.S. Pat. No. 6,785,092 B2; US 2008/0137224 A1; and by J. G. Zhu et al., “Microwave Assisted Magnetic Recording”, IEEE Transactions on Magnetics, Vol. 44, No. 1, January 2008, pp. 125-131.
In the previously proposed MAMR systems, the STO is located between the write pole and the trailing magnetic shield of the PMR write head. The STO is a multilayer film stack made up of two or more magnetic layers separated by a nonmagnetic spacer layer. One of the magnetic layers, the free layer, is designed to have its magnetization orientation oscillate in the presence of a direct electrical current perpendicular to the film planes of the film stack. In the presence of current above a critical current density, another magnetic layer, whose magnetization orientation preferably does not oscillate, acts as the “spin-polarizer” to produce a spin-polarized current at the free layer. This destabilizes the static equilibrium of the free layer's magnetization orientation, causing it to undergo sustained oscillation at frequencies useful for MAMR applications. The location of the STO between the write pole and the trailing shield may be the optimal location, if not the only possible location, to produce the necessary auxiliary field at the region of the recording layer where the write field from the PMR write head is applied. However, this location also means that the STO must be able to operate in the presence of the very large magnetic fields generated by the PMR write head. Previous STO designs, and numerical modeling of STOs, do not adequately account for the very large magnetic fields generated by the PMR write head.
What is needed is a PMR system with a STO for MAMR that is located between the write pole and the trailing shield and that operates optimally in the presence of the very large magnetic fields generated by the PMR write head.